Secondary batteries presently in wide use include lead batteries and nickel-cadmium batteries wherein the single-cell voltage is about 2 V, and an aqueous solution is used. In recent years, efforts are made to investigate and develop secondary batteries of high energy density which give a high single-cell voltage of at least 3 V and include a negative electrode of lithium. However, when lithium is used which reacts with water or the like, aprotic electrolytes must be used since aqueous electrolytes are not usable. Although polar organic solvents are presently in wide use, a majority of these solvents have a low boiling point (high vapor pressure) are inflammable and therefore involve the likelihood of staining neighboring members and ignition or firing due to a leak or break and the hazard of explosion due to erroneous use of overcharging. Furthermore, repeated discharge and charge of the secondary battery as contemplated form dendrite on the negative electrode, entailing the problem of reduced discharge-charge efficiency and short-circuiting between the positive and negative electrodes. Accordingly, many reports are made on the development of techniques for improving the discharge-charge efficiency of the negative electrode and the cycle life by inhibiting dendrite. Proposed in these reports are, for example, use of a methylated cyclic ether solvent as the solvent for battery electrolytes (K. H. Abraham et al. in "Lithium Batteries," J. P. Gabano, editor, Academic Press, London (1983)), a method of forming an ionically conductive protective film at the Li interface by adding polyethylene glycol, polypropylene glycol, polyethylene oxide or like additive to an electrolyte system (Journal of Power Sources, Vol 12, No. 2, pp. 83-144 (1984) and Unexamined Japanese Patent Publication SHO 60-41773), a method of inhibiting Li dendrite by alloying an electrode per se with Al (Unexamined Japanese Patenmt Publication SHO 59-108281).
On the other hand, M. Armand and N. Duclot disclose a novel secondary battery of high energy density incorporating a thin-film polymer electrolyte in Laid-Open French Patent Publication No. 2442512 and European Patent No. 13199. Yao et al. (J. Inorg. Nucl. Chem., 1967, 29, 2453) and Farrington et al. (Science, 1979, 204, 1371) generally describe inorganic ionically conductive solids. These solids, which are powdery, must be pelletized by a high-pressure press for fabrication into batteries. This offers a great obstacle against productivity, uniformity, etc. The pelletized solid is hard and brittle, is therefore difficult to make into a thin film of increased area, and requires a great pressure when to be adhered to the active electrode substance, so that the procedure has problems in work efficiency and adhesion. Furthermore, the solid encounters difficulty in following and compensating for variations in the volume of electrode materials during the operation of the battery and has the hazard of breaking the electrolyte. Sequlir et al. (Extended Abstracts, 163rd Meeting Electrochemical Society, 1983, 83, 751, Abstract, No. 493) describe a battery of novel design including a solvent-free thin-film polymer electrolyte, stating that the electrolyte is usable at a medium temperature of about 100.degree. C. as determined by testing. However, the conductivity at room temperature is as low as 10.sup.-6 .about.10.sup.-7 S/cm and is insufficient.
P. M. Blonsky et al. (J. Am. Chem. Soc., 106, 6854, 1984) state that polyphosphazene (MEEP) is useful as an electrolyte for electrochemical batteries. However, they merely disclose data as to a.c. conductivity in the range of from 30.degree. C. to 97.degree. C. and have not effected discharge and charge with d.c.
Further Blonsky states in the thesis for his doctorate (University Microfilms International Dissertation Information Service, 8610511, Northwestern University, PH. D. 1986), page 71 that the lithium of MEEP/CF.sub.3 SO.sub.3 Li and LiBF.sub.4 compounds has a transportation value of about 0.4 to about 0.2.
To ensure the movement of ions under d.c. conditions, the selective movement of the contemplated ion is of great importance because although the number of contemplated ions remains almost unchanged since they are constantly supplied by the electrode material, counter ions, which are not supplied from the counter electrode, are very small in number in the vicinity of the counter electrode, consequently resulting in very low electric conductivity. This tendency becomes more pronounced under a higher electric field with the lapse of time. However, with solid electrolytes wherein the solubility and diffusion mobility of salts are smaller than liquids, the transportation value of contemplated ions should ideally be 1 and is preferably approximate to 1 to the greatest possible extent. Thus, organic high polymer solid electrolytes have the advantages of being easily workable and flexible, are expected to compensate for the drawback of inorganic solid electrolytes and therefore appear very promising, whereas they have different problems to overcome for actual use. Additionally, whether the battery fabricated operates as desired as such can not be judged solely from the properties of the individual components thereof, but the battery must be tested for the judgment. For example, the problems to be encountered are the interface impedance between the components, current efficiency, resistance to redox, stability with time, etc.
An object of the present invention is to eventually provide an all solid-state lithium secondary battery with use of a compound which functions normally as a second battery electrolyte of high energy density at usual ambient temperatures from the viewpoints of:
(1) soiling and hazard due to leakage of liquid and breakage, PA1 (2) reduced efficiency and short-circuit due to the formation of dendrite, and PA1 (3) conductivity and transportation value at room temperature. PA1 i) [N=P(E).sub.2 ] is an oligoethyleneoxypolyphosphazene having a sulfone group and comprising a desired arrangement of segments represented by the following formulae (I), (II) and (III), or a mixture of such polyphosphazenes. ##STR2## ii) X is an anion. iii) M is a metal from Groups I and II in Periodic table of the chemical elements. PA1 iv) R and R' are each methyl, ethyl or propyl. PA1 v) h and k are each the average number of repeating ethyleneoxy units and are in the range of 0.ltoreq.h.ltoreq.15 and 0.ltoreq.k.ltoreq.22.5, respectively. PA1 vi) a is the ratio of (LiX) to [N=P(E).sub.2 ] and is a real number of about 0.001 to about 4. PA1 vii) l, m and n are each 0 or a positive integer in the range of 3.ltoreq.b=l+m+n.ltoreq.200000, and l+n.noteq.0.